1. Field of the Invention
The present invention relates to a toner for electrostatic image development which is employed in electrophotographic methods, electrostatic recording methods, and electrostatic printing methods.
2. Description of Related Art
Various electrophotographic methods have been disclosed in, for example, U.S. Pat. No. 2,297,691, Japanese Patent Application, Second Publication No. Sho 42-23910, and Japanese Patent Application, Second Publication No. Sho 43-24748; commonly, an electrostatic latent image is formed on an electrostatic latent image bearing medium such as a photoconductive photosensitive medium or the like by means of charge or light exposure, and then this electrostatic latent image is developed employing a toner composition containing a colorant in a binder resin, and the resulting toner image is transferred to a support medium such as transfer paper or the like and fixed, and a visible image is thus formed.
Apparatuses employing such an electrostatic image development method include copying machines, printers and the like. The processing speed varies depending on the manufacturer and type, but apparatuses having processing speeds corresponding to 30 sheets per minute in the case of office printers or about 60-100 sheets per minute in the case of office copying machines, when converted to printing speed on A4 size paper, are common. Due to increases in the speed of the developing apparatuses, conventional developing apparatuses have recently been replaced by high-speed apparatuses which have a processing speed corresponding to 100 sheets per minute when converted to a printing speed on A4 size paper in a longitudinal direction and 140 sheets per minute when converted to a printing speed on A4 size paper in a lateral direction, and which have fixing speeds of 30 m/min.
Due to power saving measure, the heat applied to the fixation of the toner has been reduced steadily, thereby reducing the heat applied from the heat roll during the fixation and shortening the pressing time due to the heat roll, thus making good sharp melting properties and low temperature fixing properties indispensable.
Also in high-speed apparatuses, there is a high possibility that the developed image makes contact with the heat roll for a long period of time if a problem occurs and that it will be exposed to excess calories, thus requiring sufficient anti-offset properties at high temperatures.
Accordingly, the toner for electrostatic image development employed therein requires a proper level according to the service conditions in electrical properties such as frictional charging properties and electric resistance related to the development and transfer properties, thermal properties related to fixation and anti-offset properties, and mechanical properties as powders such as fluidity, hardness and the like.
As the resin material for the powdered toner, for example, there have hitherto been investigated polyester, polystyrene, styrene-(meth)acrylic ester copolymer, styrene-butadiene copolymer, epoxy resin and the like, and various proposals have been made for the design of such resins depending on the use thereof.
With respect to resins for toners for fixing employing a heat roller, a lot of design examples have been proposed for the purpose of improving the fixation and anti-offset properties. To improve viscoelastic behavior during the melting by heating or to suppress changes in melt viscosity due to changes in temperature, various techniques such as enlargement of the molecular weight distribution, provision of a crosslinked structure, application of a rubbery elastic material and the like have been studied.
Due to energy reductions during the heat roll fixation or the enhancement of requirements for fixation properties at low temperatures accompanying the speeding up the apparatus, polyester resin has attracted special interest recently.
In research to date, it has been widely known that polyester resin may be employed as a resin for heat roller fixation. This is the case in, for example, Japanese Patent Application, Second Publication No. Sho 52-25420, Japanese Patent Application, Second Publication, No. Sho 53-17496, Japanese Patent Application, Second Publication No. Sho 55-49305 , Japanese Patent Application, First Publication No. Sho 55-38524, Japanese Patent Application, First Publication No. Sho 57-37353, Japanese Patent Application, First Publication No. Sho 58-11952, and the like.
However, in conventionally proposed inventions, when anti-offset properties are improved, it is not possible to avoid increases in the softening point or molecular weight of the resin, thereby degrading the anti-offset properties at low temperatures and fixation properties at low temperatures. On the other hand, when the anti-offset properties at low temperatures and fixation properties at low temperatures are improved, the softening point or molecular weight of the resin is lowered, thereby degrading the anti-offset properties at high temperatures and resistance to blocking.
A toner which reconciles these properties, which are in conflict with each other, with good balance is required. However, since properties such as anti-offset properties, fixation strength and charge characteristics vary drastically depending on the electrostatic image developing apparatus employing the toner and service conditions, design of the properties suited for these purposes is required.
Means for simply measuring the properties such as thermal characteristics and viscosity characteristics of the resin includes, for example, a constant load extrusion type capillary rheometer.
The constant load extrusion type capillary rheometer is employed for measuring the viscosity resistance in situations in which melt passes through a capillary tube, and specific examples thereof include Flow Tester xe2x80x9cCFT-500xe2x80x9d manufactured by Shimadzu Corporation.
The structure of the cylinder portion of this measuring apparatus is shown in FIG. 1.
In the drawing, the reference symbols 1 denotes a piston, 2 denotes a cylinder, 3 denotes a heater, 4 denotes a die, 5 denotes a die press, and 6 denotes a sample, respectively.
With regard to measurement by the temperature elevating method using this apparatus, measurement can be carried out continuously through out processes in which the sample 6 passes through a solid range, a transition range, a rubber elasticity range, and a fluidized range when testing while elevating the temperature at a fixed speed with the passage of the testing time. By employing this apparatus, the shear rate and viscosity at each temperature in the fluidized range can be simply measured.
The flowing curve by the temperature elevating method is shown in FIG. 2.
The range AB (softening curve) indicates a stage where the sample 6 is deformed by compression and inner pores are slowly reduced.
The point B is a temperature at which the inner pores disappear to form a transparent medium or phase having uniform appearance while maintaining a non-uniform stress distribution, and indicates an inflection point. This temperature is defined as a softening temperature Ts.
The range BC (terminating curve) indicates a range where the position of a piston 1 does not clearly change within a limited time and the sample 6 begins to flow through a die 4, and includes the rubber elasticity range of the sample 6. For a crystalline polymer, this range is short and the softening temperature shows a value close to a flow beginning temperature described hereinafter.
The point C indicates a temperature at which the sample 6 begins to flow through the die 4 as a result of a reduction in viscosity, and this temperature is defined as a flow beginning temperature Tfb.
The range CDE (flowing curve) indicates a range where the sample 6 flows through the die 4. In this range, in general, an irreversible viscous flow occurs.
The melting temperature Txc2xd by the xc2xd-process indicates a temperature at the half point of a piston stroke between Tfb and a flow ending temperature Tend in the flowing curve.
Any of the softening temperature Ts, flow beginning temperature Tfb, melting temperature Txc2xd by the xc2xd-process and flow ending temperature Tend defined herein exert an influence on the anti-offset properties and fixation properties at low temperatures during the fixation. Among these temperatures, the softening temperature Ts and flow beginning temperature Tfb contribute remarkably to the fixation properties at low temperatures and anti-offset properties at low temperatures, while melting temperature Txc2xd by the xc2xd-process and flow ending temperature Tend contribute remarkably to the anti-offset properties at high temperatures and pulverizability.
An invention made on the basis of the viscosity characteristics obtained by such a measuring method includes, for example, Japanese Patent Application, First Publication No. Hei 11-190913. The toner described in this publication is superior in anti-offset properties, but it is difficult to obtain sufficient fixation properties when employing the high-speed apparatus described above. That is, a polyester has still not been obtained which can be sufficiently employed in a high-speed apparatus whose fixing speed exceeds 20 or 30 m/min and simultaneously attain the anti-offset properties and fixation properties at low temperatures within a wide temperature range.
In the Examples of the above publication, the image of the toner is evaluated employing a high-speed copying machine (FT8200, manufactured by Ricoh Co., Ltd.), but the processing speed of the copying machine is 80 sheets per minute, when converted to printing speed on A4 size paper, and the fixing speed is about 16 m/min.
Additionally, in order to provide releasing properties from the heat roller during fixing, and in order to prevent the generation of offset, techniques have also been developed in parallel in which a releasing agent is included in the toner. To date, attention has centered on synthetic waxes such as polypropylene wax, polyethylene wax, and the like; however, examples have been disclosed in which a natural wax, such as montan wax, carnauba wax, rice wax, and the like have been employed, in Japanese Patent Application, First Publication No. Hei 1-238672, Japanese Patent Application, First Publication No. Hei 3-5764, and Japanese Patent Application, First Publication No. Hei 5-119509.
With respect to charge control agents, as well, various such agents have been considered, and a positive-charge charge control agent or a negative-charge charge control agent is selected depending on the development method and the polarity of the photosensitive medium. For example, nigrosine dyes and quaternary ammonium salt compounds and the like are known as charge control agents which may be employed in positively charged toner which is employed in machines using high speed and highly durable selenium photosensitive media. Examples in which such positive-charge charge control agents are used singly or in combination are disclosed in, for example, Japanese Patent Application, First Publication No. Hei 1-259371, Japanese Patent Application, First Publication No. Hei 3-7948, Japanese Patent Application, First Publication No. Hei 5-119509, and Japanese Patent Application, First Publication No. Hei 10-246991.
However, a discovery which exhibits all the characteristics required in the developing methods described above, for example, electrical properties such as frictional charging properties and electric resistance related to the development and transfer properties during the high-speed printing, thermal properties related to the fixation and anti-offset properties, and mechanical properties as powders such as fluidity, hardness and the like has not been disclosed in any of the referenced publications.
There have recently been demands for reduction of harmful substances generated from printers and copying machines, and the heavy metals contained in the toner in view of protection of the environment. With respect to the harmful substances generated from the printer and copying machine, impurities contained in the toner evaporate during the thermal fixation and are released into the environment. However, conventional toners were not designed with sufficient consideration of the content or generation of the harmful substances. Furthermore, some heavy metals or harmful substances of specific type and composition contained in the toner are liable to cause contamination of non-image portions and scattering of toner during printing, and consideration of them should be required in view of the printing characteristics of the toner. Consideration should also be given to countermeasures against environmental pollution and printing quality when compared with conventional printers and copying machines because the amount of the toner to be thermally fixed per unit of time is large in apparatuses for high-speed printing. However, a toner having sufficient countermeasures for these points has not yet been invented.
An object of the present invention is to provide a toner for electrostatic image development which reconciles anti-offset properties and fixation properties and is superior in resistance to abrasion and peeling of the fixed image in uses where the development and fixation are conducted at a wide range of fixing speeds, particularly at high speeds which exceed 20 or 30 m/min.
Another object of the present invention is to provide a high-durability and long-life toner for electrostatic image development, which exhibits stable development and transfer properties without changing the frictional charging properties and electric resistance during high-speed printing when being employed as a toner for a two component developer, particularly printing at high speeds which exceed 20 or 30 m/min.
Still another object of the present invention is to provide a toner for electrostatic image development, which exhibits good fluidity during high-speed printing when employed as a toner for a two component developer, particularly during printing at high speeds which exceed 20 or 30 m/min, so that the toner is quickly miscible with a carrier after being fed to the printer and can quickly reach a predetermined charge amount, whereby it is capable of forming a printed image with high image quality and high definition, which is free from fogging.
A further object of the present invention is to provide a toner for electrostatic image development in which the content of heavy metals and polycyclic aromatic hydrocarbons is suppressed to as low as possible, and which releases as small an amount of harmful volatile components as possible during high-speed printing, particularly during printing at high speeds which exceed 20 or 30 m/min, so that it is capable of forming a printed image with high image quality and high definition, which is free from fogging and scattering of toner.
As a result of diligent research with the object of solving the problems described above, the present inventors have obtained the present invention.
That is, the present invention provides a toner for electrostatic image development comprising a polyester resin, a colorant, and a releasing agent, wherein the flow beginning temperature Tfb of the toner as measured by a constant load extrusion type capillary rheometer is within a range of 70-105xc2x0 C. and the flow ending temperature Tend is within a range of 120-144xc2x0 C.
The toner for electrostatic image development according to the present invention can attain good fixation properties and anti-offset properties within a wider temperature range in uses where the development and fixation are conducted at a wide range of fixing speeds, particularly at high speeds, and is superior in resistance to abrasion and peeling of the fixed image. Furthermore, sufficient mechanical strength is present at the same time, so that there is resistance to abrasion with the carrier inside the developing apparatus, and it is thus possible to conduct printing of images with high density and high definition, which are free from fogging without leaving spent carrier or scattering of toner.